The WiFi standard strives to define the low layers of the OSI (Open Systems Interconnection) model. These layers include a physical layer called PHY and two sub-layers relating to the data link layer of OSI model:                a Logic Link Control or LLC layer, and        a Media Access Control or MAC layer.        
According to the 802.11n version of the WiFi standard, it is possible to aggregate several data packets at the MAC layer level in grouping together certain pieces of information associated with these data packets. The transmission of grouped information thus increases the efficiency of transfer of the payload information. This aggregating mechanism is illustrated, for example, in FIGS. 1B and 1C as compared with transmission without aggregation of data packets illustrated in FIG. 1A.
Thus, if no aggregating mechanism is implemented, the pieces of data are exchanged between the different devices of the BSS as follows:                acquisition of the channel 111;        transmission of a frame formed by a first PHY header referenced H_PHY 121, and a first data packet MPDU1 (MAC protocol data unit) comprising:                    a MAC header referenced H_MAC 131;            a MAC service data unit referenced MSDU 141;            a frame check sequence or frame integrity check sequence referenced FCS 151;                        acquisition of the channel 112;        transmission of a frame formed by a second PHY header, referenced H_PHY 122, and a second data packet MPDU2 comprising:                    a MAC header referenced H_MAC 132;            a MAC service data unit referenced MSDU 142;            a frame check sequence referenced FCS 152;                        etc.        
If the transmission implements an A-MSDU (aggregated MAC service data unit) type of aggregation as illustrated in FIG. 1B, the data packets coming from the level 3 layer of the OSI module are aggregated in a single MPDU data packet. The pieces of data are therefore exchanged between the different devices of the BSS as follows:                acquisition of the channel 11;        transmission of a frame formed by a PHY header, referenced H_PHY 12, and an MPDU data packet comprising:                    a MAC header referenced H_MAC 13;            two MAC service data units referenced MSDU 141 and 142;            a frame check sequence referenced FCS 15.                        
If the transmission implements an A-MPDU (aggregated MAC protocol data unit) type aggregation as illustrated in FIG. 1C, one and the same PHY header is used to transmit several data packets addressed to one and the same piece of equipment:                acquisition of the channel 11;        transmission of a frame formed by a PHY header referenced H_PHY 12, and an A-MPDU aggregated packet formed by two data packets MPDU1 and MPDU2 each comprising:                    a MAC header referenced H_MAC 131, and H_MAC 132 respectively;            a MAC service data unit referenced MSDU 141, and MSDU 142 respectively;            a frame integrity check sequence referenced FCS 151, and FCS 152 respectively.                        
The aggregation makes it possible to pool the acquisition of the channel, the PHY header or the MAC header or again to make different combinations.
In particular, the A-MPDU type aggregation makes it possible to pool the information of the PHY layer (H_PHY header 12 of FIG. 1C) while individualizing the integrity check of each data packet (FCS 151 and FCS 152 of FIG. 1C). Each data packet (MPDU1, MPDU2) therefore comprises its own MAC header (H_MAC 131, H_MAC 132) and its own frame integrity check (FCS 151, FCS 152).
This pooling of the PHY header and this differentiation of the MPDU data packets offers an efficient compromise. Indeed, if the transmission channel changes greatly from what can be determined through the PHY header, then only the last MPDU data packets will be lost because of divergence between the estimated and real coefficients of the channel. And although the 802.11 channel shows relatively slow variations, there is a risk that this situation will occur all the more frequently as the size of the frames carrying the A-MPDU aggregated packets is great.
The gains in throughput rate obtained through this A-MPDU type aggregation technique however amply warrant its use.
Besides, since the MAC throughput rate requirements are increasingly high, the frequency band used for the transmissions in a WiFi network have been increased, in concatenating several radiofrequency channels at the level of the PHY layer.
Thus, the 802.11a, b, g WiFi standards propose the use of a 20 MHz frequency band while the 802.11n WiFi standard proposes the use of a 40 MHz frequency band (where 40 MHz frequency band corresponds to a concatenation of two 20 MHz radiofrequency channels without overlapping). The standards currently being standardized propose the concatenation of up to four 20 MHz or 40 MHz channels (which may be adjacent or disjoint) to enable a transmission of data on an even wider band, for example 80 MHz.
FIG. 2 presents an example of transmission of data on a concatenated channel formed by a primary channel 21 and a secondary channel 22 according to the 802.11n standard.
To ensure transmission on a plurality of channels, the CSMA-CA (carrier sense multiple access-collision avoidance) access mode as described in the 802.11-2007 standard, paragraph 9.1 “MAC architecture”, 9.1.1 “DCF”, is implemented on the primary channel 21. This CSMA-CA mechanism provides for a sharing of access to a radiofrequency channel according to a principle known as that of contention: each device must listen to see if the channel is free (i.e. that no signal is being sent/received in this channel) for a variable duration corresponding to an AIFS (arbitrary inter-frame space) duration and a random waiting period (denoted as B for backoff) before transmitting data. This listening phase (based on a physical layer mechanism called CCA or clear channel assessment) gives access to the primary channel (acquisition of the channel 211) and is therefore implemented solely on the primary channel 21.
On the secondary channel 22, a verification is made only of non-occupancy for a specific duration known as PIFS (PCF inter-frame spacing), varying between 25 and 36 microseconds according to the IEEE 802.11n standard, using especially the CCA mechanism. The secondary channel 22 is pre-empted after the PIFS duration (if no exchange is detected during this period), implying that a device that needs a primary channel 21 and secondary channel 22 in order to send on a broader frequency band will obtain priority access to the secondary channel 22 if the primary channel 21 is free (because the PIFS duration is smaller than the duration of acquisition of the primary channel 21).
Data packets can thus be transmitted on a broader frequency band corresponding to the concatenation of the primary channel 21 and secondary channel 22 (at the PHY layer).
This technique of concatenation of the channels, which considerably increases the available band, makes it possible to obtain very high transmission throughput rates, especially when it is combined with a A-MPDU-type aggregation implemented at the MAC layer level.
FIG. 3 provides a more precise illustration of this A-MPDU type aggregation mechanism implemented on a concatenated channel C formed by four radiofrequency channels, each having a 20 MHz bandwidth and denoted as a primary channel 31, a secondary channel 32, a tertiary channel 33 and a quaternary channel 34. As described here above with reference to FIG. 2, the acquisition of the channel is implemented solely on the primary channel 31 when several channels are concatenated. The other channels 32, 33 and 34 are pre-empted.
The data packets MPDU1 to MPDU5 of FIG. 3, each comprise a MAC header (referenced H_MAC), a MAC service data unit (referenced MSDU1 to MSDU5), and a frame integrity check sequence (denoted as FCS) forming an A-MPDU aggregated packet. These data packets MPDU1 to MPDU5 are spread over all the radiofrequency channels of the concatenated channel C.
As already described, a same header PHY is used to transmit several data packets addressed to a same device. The PHY header (referenced H_PHY) is therefore duplicated on the different radiofrequency channels 31 to 34, and sent synchronously. There is therefore no spreading of this information. In this way, the devices using the non-primary channels can know the length of the frame without needing to consider all the radiofrequency channels. Reception can be done correctly only if the PHY header has truly been received.
The frame formed by the headers H_PHY and the A-MPDU aggregated packet is then sent on the concatenated channel C.
In addition to giving very high transmission throughput rates, the concatenation of the radiofrequency channels combined with the implementation of an A-MPDU type aggregation limits transmission errors. Indeed, as illustrated in FIG. 3, the data packets spread over all the radiofrequency channels of the concatenated channel C last for a shorter time (in terms of transmission time) and are therefore more resistant to the variations of the channel.
Unfortunately, the concatenation of radiofrequency channels also has drawbacks.
Indeed, the probability of having a collision of data packets after a successful contention phase depends not only on the number of stations of the basic service set (BSS) but also on the traffic and the distance between an access point and the stations that are associated with it. This probability of collision is not negligible when a plurality of radiofrequency channels is used. Indeed, it is possible that a device of the BSS sending in only one radiofrequency channel (a main channel or another channel) will obtain a backoff period B that finishes at the same time as the backoff period allocated to a device sending in several radiofrequency channels. The two devices will then consider the radiofrequency channel to be free and send data simultaneously, and this will lead to a collision of the data packets. This is a flaw in the CSMA/CA mechanism.
The problem of the concealed stations is also responsible for numerous collisions. It may be recalled that, by definition, all the stations are within range of the access point (AP) which federates the BSS. On the contrary, not all the stations are necessarily within range of one another. It may therefore happen that a station transmits data to the AP (while perfectly complying with all the imposed mechanisms) while another station, which is outside its range, is already transmitting data to the AP. The AP therefore loses data coming from both stations because of jamming induced by the first station, which in this case is called a “concealed” station.
Furthermore, when the band used is extended, the probability of collision (after successful acquisition of the desired channels) is doubled, tripled or even quadrupled depending on the number of radiofrequency channels over which the frame extends. There is therefore a high risk of collision.
Besides, even in using an A-MPDU type aggregation mechanism, a single collision on one of the radiofrequency channels can lead to the loss of the entire frame since all the headers will be corrupted. If the effect of the collision is far too great to be corrected by the mechanisms of the PHY layer, the data packets will be wrongly demodulated and the integrity check will give a negative result. It therefore becomes necessary to retransmit the data packets, and this consumes bandwidth and reduces the efficiency of the system.
There is therefore a need for a novel technique of transmission on multiple channels using a mechanism of aggregation of the data packets, enabling the risk of collision to be limited.